Sometimes fanon gets confirmed later, or just adopted because it is actually a quite nice take on the story.

Like Vault Tec being the ones to drop the nukes, or the Xenomorphs being made intentionally as a weapon.

Sometimes the fanon is basically canon but there's just not textual confirmation.

Like Jon Snow being the son of Lyanna Stark

But most of the time fanon either fulfills a need for the story to be better than it was, or it just refers to established conventions for fan fiction purposes.